1. Field of the Invention
The invention relates to an optical data carrier as well as to a method and signal being suitable for recording track data onto an optical data carrier.
2. Discussion of the Background
FIG. 1 shows a schematical drawing of a well known optical data carrier, a CD (compact disk) 1. The CD 1 comprises a transparent substrate layer 2 and a reflective information layer 3. The CD 1 contains a spiral shaped path of successive shallow depressions 4, also called pits, in the reflective information layer 3. Encoded information is stored in the lengths of the pits 4 and in the distances between the pits 4. During playback of the CD 1 a scanning light spot 5 which is focused by an objective lens 6 is diffracted by the pits 4 in the reflective information layer 3. The optical power that is diffracted back into the objective lens 6 is modulated according to the information encoded within the reflective information layer 3. The optical power that is diffracted back into the objective lens 6 is converted into a photo current also called high frequency (HF) signal.
Generally, the arrangement of the optical system which generates the scanning light spot and which comprises the objective lens 6 is optimized for predetermined thickness values of the substrate layer 2. This causes a problem if the thickness values of a CD do not fulfill the thickness assumptions made. In this case, an undesired effect called “spherical aberration” occurs: Normally, the irradiation distribution of the scanning light spot 5 shows a distribution referenced with the reference numeral 7 (FIG. 2a). In the case of spherical aberration, the irradiation distribution of the scanning light spot 5 shows a distribution referenced with the reference numeral 8 (FIG. 2b). The irradiation distribution 8 leads to a high “jitter” (the standard deviation of time length variations between leading and trailing edges of specific pits 4 or lands (areas between the pits 4), measured at reference scanning velocity). A high jitter, however, may lead to a weak high frequency signal which can not be properly processed.
The problem described above concerns all types of CDs: the CD-DA (Compact Disc Digital Audio), the CD-ROM (Compact Disc Read Only Memory) or the SA-CD (Super Audio Compact Disc).